Diluvium. 



143 



the wreck of a granite ledge. A view of this rock will be given 

 farther on. In Plymouth County, except at its northern part, the 

 granite rarely appears, and but seldom forms a cliff even fifty feet 

 high. Every thing, indeed, is buried by diluvium ; and, as the 

 streams are few and small there, it is extremely difficult to ascertain 

 what is its geology, except to say that it is diluvial. 



The diluvium of Plymouth and Barnstable counties consists al- 

 most entirely of white sand, some pebbles, and a very large number 

 'of bowlders of primary rocks. These bowlders consist chiefly of 

 granite, sienite. and gneiss, with occasional masses of gray wacke 

 conglomerate, compact feldspar, and porphyry. They all correspond 

 with the rocks found in place along the coast, in the vicinity of Bos- 

 ton, and on Cape Ann : and no one, it seems to me, can see the marks 

 of degradation along that coast, who will not be convinced that a 

 large portion of the pebbles and bowlders of Plymouth and Barnsta- 

 ble counties, must have come from thence. Along the range of ele- 

 vated, and for that part of the State, even mountainous land, which is 

 colored as granite on the map, the bowlders are so enormously large r 

 and so thick, that I cannot believe they have been ever removed far 

 from their native beds. They are sometimes from 10 to 20 and even 

 30 feet in diameter, and frequently occupy nearly the whole surface • 

 so that one can hardly persuade himself, when he examines them 

 from a little distance, that they are not genuine ledges. Indeed, I 

 have repeatedly been deceived by their appearance, until I had gone 

 among them, and ascertained that they were detached bowlders. On 

 the road from Sandwich to Falmouth is perhaps as striking an exhi- 

 bition of this phenomenon as in any place, unless it be in the western 

 part of Martha's Vineyard, in Tisbury and Chilmark. The same 

 appearance is striking, also, in Brewster, on the Cape : and I doubt 

 not that genuine ledges of granite may be found in those places ; al- 

 though (with the exception of Brewster perhaps,) I did not make the 

 discovery. I have been informed, however, that rocks in situ, do 

 exist in Dennis. But I have been so often deceived in this matter in 

 that region, that I dare not state any thing as fact concerning it, 

 which I have not carefully examined with my own eyes. At any 

 rate, I cannot believe that bowlders so large and numerous have been 

 removed many miles ; for powerful as has been the diluvial current 

 in the eastern part of the State, I have seen no well ascertained in- 

 stance where whole mountains have been torn up and transported, as 



